There are many electronic devices that require or otherwise employ the conversion of DC electricity to AC electricity, typically by operation of an inverter. Such devices include, for example, solar inverters, variable frequency drives, and uninterruptible power supplies.
Solar inverters may be used, for instance, to convert the DC output of solar photovoltaic modules to an AC output that can be utilized by AC devices, or provided to an electric power grid. Uninterruptible power supplies typically use batteries to provide a DC source, which is then inverted to produce an AC output. Variable frequency drives typically rectify AC to produce DC, perform a DC to DC conversion to obtain a desired DC voltage level, and then invert this DC voltage to generate an AC voltage of desired frequency. Other DC-to-AC inverter applications will be apparent. The parameters and quality of the AC output provided by the inverter may vary depending on the particular demands of the target application. For instance, the AC output may range from a relatively crude square wave that is usable in non-critical applications, to a relatively smooth sine wave having a quality comparable to that generated by electric utilities or useable in critical applications (e.g., medical or military).
Conventional DC-to-AC inverters typically use pulse width modulation to simulate the electromotive force of alternating current. High speed electronic switching is usually employed to turn direct current on and off. The width of the pulses may be varied to simulate the effect of alternating current at a particular location in its sine wave. The polarity of these pulses may be alternated to then simulate the effect of the positive and negative characteristics of a sinusoidal waveform, through an H-bridge or similar mechanism, for example.
In any such cases, there are a number problems associated with conventional DC-to-AC inverter designs. For instance, the high-speed switching process may generate undesired harmonic frequencies. In addition, conventional inverter technologies are typically expensive and may contain complicated electronic circuits and many components, further increasing cost. Such high cost can be of particular concern in solar photovoltaic applications, as the cost of the inverters may represent a significant percentage of the overall photovoltaic system cost.
There is a need, therefore, for techniques that allow for efficient, cost-effective DC-to-AC conversion.